This invention relates to an apparatus and system for inspecting a surface transparent to infrared radiation for defects. More particularly, this invention relates to a portable apparatus and system for inspecting such surfaces.
The need for data regarding the performance and long term reliability of a photovoltaic solar array has created the need for inspection equipment to aid in this task. Flat plate (non-concentrating) photovoltaic arrays currently consist of commercially manufactured panels with electrical strings of silicon wafers, gallium arsenide surfaces or the like, wired in a series-parallel arrangement and either encapsulated or mounted under some type of protective cover plate.
Typical damage to this type of array under normal, long-term operating conditions might consist of: fractured solder joints, encapsulant delamination, cracked cells or split interconnects. All above noted defects can be characterized as mechanical or macro-defects and can, under laboratory conditions, be located using conventional microscope techniques. Of these defects, cracked silicon cells are most prevalent, difficult to identify and the most time dependent. A crack in the silicon wafer which crosses a surface conductor can electrically open a series string in a module and stop the flow of current through that string.
Several useful semiconductor materials including silicon, which are opaque in visible light, are transparent to near-infrared radiation. The transmission of silicon cuts-on sharply between 0.9-1.1 microns and extends out to beyond 2.5 microns before significant drop-off. Infrared microscopy makes possible the observation of the internal structure of silicon semiconductor devices, including evaluation of the substrate and lead attachments on the back side of the wafer. This technique is routinely used by manufacturers of semiconductor devices.
This technique has utility in locating cracks in relatively large silicon solar cells. Observation of cracks at visible wavelengths is limited to only that portion of the crack lying in the surface plane of the wafer. In the absence of significant surface displacements of the cell adjacent to the crack, the visual information needed to locate and identify a crack is severely limited. Resolution losses through an optical system further reduce the possibility of defect identification.
The number of panels and cells that make up a large field installation along with the physical size of the array render currently available inspection techniques unsatisfactory. One must tolerate long inspection times, operator discomfort and difficulty in reaching all parts of the array surface, in order to achieve a statistically representative sample of inspection data. Accordingly, such an inspection device must address these problems, should be portable and should be universally adaptable to the currently used array frame mechanical configurations.